Thomas Jang was born in Neujiji, Suwon, Gyeonggi-do (now, Yodang-ri, Yanggam-myeon, Hwaseong-si). He was a cousin of St. Joseph Jang Ju-gi who was martyred in 1866. He became a Catholic with his cousin Joseph Jang.

After that they dedicated their lives to the Church. They had to move from one place to another to practice their religious life. St. Joseph Jang settled in Baeron, Chungcheong-do (now, Guhak-ri, Bongyang-myeon, Jecheon-si, Chungbuk). Thomas settled in Baeti, Jincheon (now, Yangbaek-ri, Baekgok-myeon, Jincheon-gun, Chungbuk). At that time the catechist Simon Jang, who may be a relative of Thomas Jang, was living there.

In Baeti, Thomas Jang observed his religious duties faithfully, and educated his only son to be a good Catholic. Friends and relatives all called him `a good man` because he was gentle and good by nature.

When the Byeongin Persecution broke out in 1866, Thomas Jang heard that many Catholics were arrested. But he decided to stay where he was, and follow God`s will instead of escaping. Very soon he and his family were arrested by the police of Cheongju who broke into his house.

They were immediately taken to the Jincheon government office, and Thomas Jang had to undergo a severe session of interrogation and torture. The magistrate said, "If you renounce the Catholic religion, I won`t kill you. I will return your fortune to you. You will have a good life." He replied with determination, "I can give up my life and fortune, but I cannot betray the Catholic Church."

Before long Thomas Jang was transferred to Cheongju where soldiers were stationed. he was interrogated and tortured again. The chief commander asked him; "Will you not give up the Catholic Church?" Thomas Jang made the same reply saying, "I cannot betray the Catholic Church though I have to die ten thousand times."

Thomas Jang was sentenced to death. He was taken to the `Jangdae` (now, Nammunro 2-ga, Cheongju-si) where the military command headquarters was located. On the execution ground, seeing his god-son who was about to betray God, Thomas Jang said, "My son, you have believed in God and worshipped Him up to the present. Why do you want to save your life and lose this opportunity to die for God? How can you escape from the punishment of the Lord in the future?" Upon saying this he put his throat under the sword of the executioner and died a martyr. Thomas Jang was 51 years old.